CO2 Lasers Expose Hypersonic Flow

A two-hour flight from New York to Tokyo could someday be a reality with the development of a hypersonic transatmospheric vehicle. Such aircraft, though, would routinely face aerodynamic drag and heating on a par with conventional spacecraft developed for less-routine flights. In the early 1990s, researchers first coined the term DEAS, or directed energy air spike, to describe a method to reduce drag and heating by adding energy to the air ahead of the vehicle. One way to accomplish this would be to focus a powerful laser beam ahead of the flight path.

Marco A.S. Minucci and colleagues at the Laboratory of Aerothermodynamics and Hypersonics-CTA in São José dos Campos, Brazil, are studying the DEAS effect using optical breakdown upstream of a model that is induced by a CO2 transversely excited atmospheric pressure laser.

Taken with an SLR Nikon camera using ASA 100 Kodak color film, this open-shutter image shows a test done with airflow of Mach 7.3 and 5.5-J laser energy (single pulse). Pictured are the expected bow shock (luminous layer near model surface) and the conical flow structure (initiated by laser-induced breakdown). Both flow structures are superimposed because of the nature of the time-integrated photography technique. Airflow is from right to left.
The 100-mm-diameter test subject features the same geometry as the re-entry heat shield of an Apollo spacecraft and is fitted with piezoelectric pressure transducers and platinum thin-film heat-transfer gauges.

In collaboration with a US-based research team, the scientists are conducting the experiments in the Brazilian lab's 0.3-m hypersonic shock tunnel, which can produce airflow with Mach numbers up to 15 and air velocities approaching 5 km/s. In multimode operation, the laser can produce a single, high-energy -- 7.5-J -- 120-ns pulse. The beam, which has a 34 x 17-mm cross section, is focused 95 mm ahead of the model's front surface through a 50-mm-diameter NaCl lens with a 180-mm focal distance. The lens mount is in a beam delivery system installed in the tunnel test section wall at a 45° angle with respect to the model centerline.

The scientists synchronize the laser pulse with the shock tunnel useful test time via a time-delay generator triggered by a piezoelectric pressure transducer immediately upstream of the nozzle entrance. They use three Hamamatsu germanium photodiodes as light sensors to monitor pulse generation inside the laser head, production of the laser-induced air ignition inside the test section, and the natural air luminosity of the hypersonic, hypervelocity flow around the model.

In one test, time-lapse imaging during flow at Mach 7.3 allowed the researchers to visualize both the expected bow shock and the conical flow structure initiated by laser-induced breakdown. While the research into laser-induced DEAS is ongoing, Minucci reports that drag reduction of up to 40 percent has been detected so far, with all the luminosity gener-ated only by the flow.